Using an active silencer in a vehicle exhaust system can improve engine efficiency because of reduced exhaust back pressure. One problem with active silencers is that active silencers are often bulky and their use requires the vehicle to be specifically designed or retrofitted to accommodate the active silencer. The invention is an active silencer and a method of making the same that allows for compact, low-profile construction, thus substantially eliminating this problem.
In general, active silencers inject a canceling acoustic wave to destructively interfere with and cancel an input acoustic wave. It is typical to sense the input acoustic wave with an input microphone and the output acoustic wave with an error microphone. The input microphone supplies an input or feedforward signal to an electronic controller, and the error microphone supplies an error or feedback signal to the electronic controller. The electronic controller, in turn, supplies a correction signal to one or more loudspeakers that generate a canceling acoustic wave to destructively interfere with an input acoustic wave such that the output acoustic wave at the error microphone is zero (or at least reduced).
In a vehicle exhaust system, it is desirable to keep the speakers in an active silencer protected from the hot exhaust gases to prevent premature deterioration. One way of providing such isolation is disclosed by Bremigan in U.S. Pat. No. 5,044,464 in which two speakers are located in a chamber away from the main exhaust flow. The canceling acoustic waves are directed from the chamber to the exhaust flow. In U.S. Pat. Nos. 5,233,137 and 5,229,556, Geddes discloses a system isolating the speakers from the exhaust flow wherein a tuned chamber is ported to the exhaust flow. This system has the advantage of improving speaker efficiency over the frequency band width appropriate for the application.
In an exhaust system, it is also desirable that the active silencer does not significantly shake or vibrate during operation. Such shaking or vibration can be to a large extent due to movement of the speakers. The Bremigan and Geddes patents show a pair of speakers mounted face-to-face so the axial load of the speakers cancel one another thus eliminating structure vibrations by the speakers. However, the face-to-face speaker arrangement adds considerable bulk to the design. In particular, the dimensions of a system using a face-to-face arrangement is too tall to fit in a conventional automobile without retrofitting.
Therefore, it is desirable to have a low-profile active exhaust silencer in which the speakers are isolated from the exhaust flow.